The present invention relates to a progressive multifocal ophthalmic lens that includes an aspherical or nonspherical surface having a mean sphere and a cylinder at every point thereof and comprising a distance vision portion, a near vision portion and an intermediate vision portion therebetween.
Progressive multifocal ophthalmic lenses are now well known. They are employed for correcting presbyopia (longsightedness), and enable people wearing spectacles to look at objects over a wide range of distances without having to remove their spectacles. Such lenses typically include a distance vision portion, located at the upper part of the lens, a near vision portion at the bottom part of the lens and an intermediate portion linking the near and distance vision portions.
The front face, in other words the face directed away from the wearer of the spectacles, of progressive multifocal lenses is generally aspherical and the rear face, i.e. the face directed towards the wearer of the spectacles is toroidal or spherical in shape. This toroidal or spherical face enables the lens to be adapted to the user's particular ametropia, so that a progressive multifocal lens is only generally defined by its aspherical surface.
As is well known, an aspherical surface such as the face of a lens is generally defined by the height of all the points on it. Parameters consisting of the minimum and maximum curvatures at each point, or more frequently half of their sum, and their difference, are also employed. The half-sum and difference multiplied by a factor n-1, where n is the refractive index of the lens material, give a value for what is known as the mean sphere (or power) of the lens and the cylinder of the lens.
Families of progressive multifocal lenses are defined in which each lens of a family is characterized by the result of an addition value corresponding to the variation in mean power between the near vision portion and the distance vision portion. More precisely, the addition value, referred to by the letter A, corresponds to the variation in mean power between a point in the distance vision portion and a point in the near vision portion, these points being respectively called the distance vision power control point and the near vision power control point, and which represent the points where the line of sight intersects the lens surface for infinite vision and for reading vision.
Within a a given family of lenses, the power addition value varies from one lens to another within the family between a minimum addition value and a maximum addition value. Usually, the minimum and maximum addition values are respectively 0.5 diopters and 3.5 diopters, the addition value varying in 0.25 diopter steps from one lens to the next within the family.
In order to create progressive multifocal lenses, one generally starts by determining a line referred to as the principal meridian of progression, corresponding to the points at which the spectacle wearer's line of view intersects the lens surface, when the wearer looks at objects at different distances. Following this, the shape of the aspherical surface is defined along this meridian and in-the vicinity thereof.
Among known multifocal ophthalmic lenses, there are essentially two types of family of lens. In the first type of family of lenses, the distance between the two abovementioned control points is substantially constant and the slope or gradient of the optical power varies from one lens to another in the family. Such lenses are for example described in French patent 2,058,499 and in the two Certificates of addition thereto 2,079,663 and 2,193,989.
Regarding the second type of family of lenses, the slope or gradient of optical power along the principal meridian of progression is constant and identical for all the lenses of the family, regardless of the addition value of their powers (see Japanese patent 54-85743). In this case, the distance between the said points varies from one lens to the next within the family.
In order to characterize such types of lens, a parameter known as the effective progression length is used. In a coordinate system where the x-axis corresponds to the horizontal axis of the lens and the y-axis corresponds to an axis vertical to the periphery of the lens, the effective progression length is the distance along the y-axis for which a variation in mean power corresponding to at least 85% of the power addition value A is encountered.
Multifocal ophthalmic lenses regardless of their type, inevitably suffer from optical aberrations (astigmatism, distortion, prismatic deviations, etc.) which have a negative effect on visual comfort both under static and dynamic vision conditions.
Furthermore, after a change of lenses, and encountering lenses of a higher power addition value, the wearer of the spectacles usually needs to make an effort to physiologically adapt to them. The time needed for such adaptation can vary from one to several days, depending on the subject.
The applicant has proposed a third type of family of ophthalmic lenses which attempts: to deal with the problem of reducing the effort of physiological adaptation and the time needed to adapt when the wearer changes from a pair of lenses having a first power addition value to a pair of lenses having a second higher power addition value (French patent 2,617,989).
The applicant has also proposed, with a view to better satisfying the visual needs of persons suffering from presbyopia and to improve the comfort of progressive multifocal lenses, to adapt the form or shape of the principal meridian of progression as a function of the power addition value A.
Existing lenses do provide a satisfactory solution to static vision and foveal vision. There is nevertheless room for improvement as regards dynamic vision, in other words vision of objects that move in the field of vision due to their own movement or movement of the wearer of the spectacles. It would be also useful to reduce defects of peripheral vision. One can consider that existing progressive lenses provide quite satisfactory vision when the wearer of spectacles is looking at an object located straight in front of him, and at any distance whatsoever. In this case, the wearer's line of view is precisely along the principal meridian of progression, and the research undertaken by the applicant and disclosed in the above-cited patents does provide a high level of comfort.
Outside the region of the principal meridian of progression, vision with progressive multifocal lenses does present problems, resulting from the aspherical nature of the lenses and in particular, from the fact that the points where the line of view for peripheral vision intersects the right and left lenses do not necessarily follow horizontal and vertical curves which are identical or are at least sufficiently close to rule out all discomfort. It has been found that variations in horizontal curvature did not in fact cause notable discomfort and there was a proposition to limit variations in vertical curvature for points on the lens located along a given y-coordinate. Such differences in curvature lead to vertical prismatic effects which are not equivalent and which have a negative effect on peripheral vision.
More precisely, in French patent 2,193,989 the applicant proposed limiting the variation in vertical curvature between a given point on the lens and the point on the principal meridian of progression having the same y-coordinate, as a function of the value of the lens power addition value. This solution enabled vertical prismatic deviations to be reduced, thus improving progressive lens comfort.
Despite the advantages provided by this solution, wearers of progressive multifocal lens spectacles still experience discomfort with dynamic vision or vision in the lateral portion of the distance and near vision portions.